Stoichiometric Experiments with Alkane Combustion: A ClassroomDemonstrationDenis M. Zhilin*

Moscow Institute for Open Education, 125167, Aviacionnyj per., 6, Moscow, Russia

*S Supporting Information

ABSTRACT: A simple, effective demonstration of the concept of limiting and excessreagent is presented. Mixtures of either air/methane (from a gas line) or air/butane (froma disposable cigarette lighter) contained in a plastic 2 L soda bottles are ignited. Themixtures combust readily when air/fuel ratios are stoichiometric, but not at a 2-fold excessof any of the reactants. Safety issues and typical errors are also discussed.

KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Demonstrations, Organic Chemistry,Physical Chemistry, Hands-On Learning/Manipulatives, Alkanes/Cycloalkanes, Gases, Stoichiometry

Alkanes are notoriously unreactive in most chemicalreactions with one exception: their combustion with

oxygen is spectacular and forms the basis for this demonstrationof the concept of stoichiometry. The purpose of thedemonstration is to establish the ideal air/alkane (particularly,air/methane and air/butane) ratio that results in optimalcombustion. Propane could also be used, but was notinvestigated here. While observing the demonstration, studentsdevelop skills in writing reaction equations and in the use ofAvogadro’s law.Methane is the primary constituent of natural gas, butane can

be obtained from a disposable cigarette lighters, and propane isavailable from camping cylinders. All of these gases are widelyused in school experiments.1 Moreover, their combustion wasused2 to develop the concept of stoichiometry but thedescribed experiment was indirect: the luminosity of methaneand butane flames was compared without measuring air/alkaneratio directly.During the demonstration, the teacher prepares three

mixtures: stoichiometric; 2-fold excess of alkane; and 2-foldexcess of air. The best way is to prepare the mixtures in clear 2L plastic soda bottles. A burning splint is inserted into themouth of the bottle and the combustion process is eitherobserved or not. If there is enough time or if students arecurious, mixtures with 1.5-fold excess of each reactant can alsobe investigated. The demonstration with methane takes lessthan 10 min, and the demonstration with butane takes about 15min.

■ CALCULATIONSBefore viewing the demonstration, students carry out thecalculations needed to determine the air/alkane ratio forwhichever fuel is used. This may be done as a teacher-leddiscussion or independently, depending on the students’ ability.The reactions have the following equations:

+ → +CH 2O CO 2H O4 2 2 2

+ → +2C H 13O 8CO 10H O4 10 2 2 2

The oxygen/alkane ratio (coefficients) can be designated as α.According to Avogadro’s law, this means that α liters of oxygenper liter of alkane are needed. Because air contains 1/5 parts ofoxygen, this gives 5α liters of air per liter of alkane. Then, theportion of alkane in the gas mixture is 1/(1 + 5α), resulting inthe volume of alkane as Va = Vb/(1 + 5α), where Vb is thevolume of the bottle used to contain the air−fuel mixture. For a2 L bottle, the volumes are shown in Table 1.

■ EXPERIMENTAL PROCEDUREAir−ButaneTo prepare an air/butane mixture, a disposable cigarette lighterwith the metal housing removed is used. Using a syringe or agraduated cylinder, a volume of water equal to the desiredvolume of butane is poured into the 2 L bottle. The bottle isplugged with a stopper and inverted in a basin of water. Withthe mouth of the bottle below the surface of the water, thestopper is removed (Figure 1).

Published: February 1, 2012

Demonstration

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The water in the bottle is then displaced with an equalvolume of butane. For this, a thin plastic tube is put onto the jetof the cigarette lighter, whose metal housing has been removed(Figure 2), and the other end of the plastic tube is placed into

the mouth of the 2 L bottle, which is under the water. Bypressing the button on the head of the lighter, butane isreleased until it displaces all the water from the bottle. With thebottle still inverted and under the surface of the water, it isplugged with a stopper. The bottle is removed from the basinand is shaken for 1 min to allow thorough mixing of the fueland air.The lighting of the fuel mixture should be performed behind

a blast shield. To ignite the mixture, the bottle is set upright andthe stopper removed. A burning splint is held with tongs intothe bottle. The splint should not be held with an unprotectedhand. Either tongs are used or a heat-proof glove is worn as theflame can shoot up and cause burns.

Air−Methane

To prepare the air/methane mixture, the methane may bedelivered using either the downward displacement of watermethod discussed with butane or by the displacement of airfrom an inverted 2 L bottle. With both methods, a large syringeis filled with methane from the gas tap by placing a tubebetween the gas tap and the nozzle of a syringe. For simplicity,natural gas is assumed to be pure methane. The syringe is filledwith the necessary volume of the gas by pulling the plunger.The syringe is then inserted as far as possible into the invertedbottle so that either the water or denser air is displaceddownward as the methane is injected. The open 2 L bottleshould remain upright as the air/methane mixture is less densethan air. The bottle is plugged with a stopper and is shaken tomix the gases. The bottle is placed upside down into a metalring attached to a retort stand (Figure 3). Finally, the bottle is

unplugged and the air/fuel mixture is ignited using a burningsplint, observing the safety precautions mentioned above.A fresh 2 L bottle should be used for each fuel sample, as this

saves time during the demonstration. If the same bottle must beused, the products of combustion should be replaced with anew portion of air. For this, the gas is either blown out of thebottle by a flow of fresh air or the bottle is filled with water andthe water poured out. Smaller bottles are not recommended, asthe demonstration is not visually appealing.

■ HAZARDS

This demonstration is of the “whoosh bottle” type. Althoughthe demonstration is safe, it requires some common-sensesafety precautions.3−6 The demonstrator should wear a lab coatand safety goggles and the demonstration should be performedbehind a plexiglass blast shield. Plastic (not glass) bottleswithout cracks should be used. Further, oxygen should not beused in concentrations higher than that in air. When butane isreleased from the lighter, the jet and the head of the lighterbecomes cold; this is not hazardous, but may alarm ademonstrator who is conducting this experiment for the firsttime. It is essential that the demonstrator practice thedemonstration before presenting it to a class. The cigarettelighter or gas jet must be turned off after filling a bottle. Thefastest reactions take about 1 s with a slight whoosh and no signof explosion. Bottles with a waist (Figure 4) should be avoided.To avoid burns, the air/butane mixture should be ignited usinga burning wooden splint fixed by tongs or by hand wearing a

Table 1. Volume of Alkane Required To Prepare Air/AlkaneMixtures for a 2 L Bottle

Composition Volume Methane/mL Volume Butane/mL

2-Fold excess of alkane 333 1161.5-Fold excess of alkane 261 88Stoichiometric ratio 182 601.5-Fold excess of air 125 402-Fold excess of air 95 30

Figure 1. Displacing the water with an equal volume of butane.

Figure 2. A tube on a disposable cigarette lighter jet, with the metalhousing of the lighter removed.

Figure 3. Igniting of air/alkane mixtures.

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heat-proof glove. The air/methane mixture should be ignitedwith the splint held horizontally.

■ DISCUSSIONThe experiments show that methane and butane mixtures burnin a similar manner (Table 2). They combust when the ratio is

stoichiometric or when a 1.5-fold excess of either reactant ispresent (nonstoichiometric mixtures burn slower and morequietly). Two-fold excess mixtures do not burn at all. Thisshows the importance of stoichiometry and helps to developthis concept.

■ ERRORS AND MISTAKES

Errors and mistakes can influence the results. The followingerrors are noted. The butane in some disposable cigarettelighters may contain a propane/butane mixture that will affectthe stoichiometry. If the demonstrator inadvertently squeezesthe 2 L bottle while filling it with gas, some water will bepressed out, leading to a lower volume of gas being added. Ifthe demonstrator continues pressing the valve of a lighter afterthe water is fully displaced with butane, the volume of gas willbe larger than specified.

■ CONCLUSION

Investigation of flammability of air/alkane ratio is a safedemonstration, suitable for studying alkanes and stoichiometry.

■ ASSOCIATED CONTENT

*S Supporting Information

Notes for the demonstrator; two movies showing the air/fueldemonstrations; Excel file of the calculations. This material isavailable via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author

*E-mail: zhila2000@mail.ru.

■ ACKNOWLEDGMENTSThe author acknowledges the helpful comments of MichaelJansen, Chemistry Teacher at Crescent School, Toronto.

■ REFERENCES(1) Davenport, D. A. J. Chem. Educ. 1976, 53, 306.(2) Alexander, M. D. J. Chem. Educ. 1993, 70, 327−328.(3) Campbell, D. J. Chem. Educ. 2004, 81, 31.(4) Fortman, J. J.; Rush, A. C.; Stamper, J. E.; Waner, M. J.; Young, J.A. J. Chem. Educ. 1999, 76, 1092.(5) Senkbeil, G. E. J. Chem. Educ. 2004, 81, 31.(6) Vitz, E. Note to Campbell, J. D.; DeWees, S. J. Chem. Educ.2001, 78, 910−911.

Figure 4. Appropriate and inappropriate shape of the 2 L bottle.

Table 2. Burning of Air/Alkane Mixture

Composition Observationsa

2-Fold excess ofalkane

Does not burn inside the bottle; a small flame over themouth of the bottle is observed.

1.5-Fold excessof alkane

Burns for 1−2 s with a whoosh, with a circle of blue flamemoving up inside the bottle and blue flame jet emanatingfrom the mouth.

Stoichiometric Burns for less than 1 s with a whoosh and a circle of blueflame moving up inside the bottle and an almost colorlessflame jet emanating from the mouth.

1.5-Fold excessof air

Burns for 1 s with a very slight whoosh and almost noobservable flame. A slight flame jet may emanate from themouth.

2-Fold excess ofair

Does not burn.

aThe observations are almost similar for methane and butane.

Journal of Chemical Education Demonstration

dx.doi.org/10.1021/ed200506d | J. Chem. Educ. 2012, 89, 649−651651